Both. Although two of each will be much more musical. Or three of one and one of the other. Actually, anything where you wind up with at least four is more musical than anything with just one.
Specify "why"? Okay. Because when it comes to subs no matter which one you get they all have the same problem of room modes that always make the bass lumpy. Way too much one place, way too little the other. Even with EQ, impossible to ever get it right.
Multiple subs however go in multiple locations and produce a lot of different modes in a lot of different locations, the overall effect of which is much smoother, faster, more articulate bass. Its not even close.
So get whichever is cheaper, because then you can afford more, and more is what you want.
Specify "why"? Okay. Because when it comes to subs no matter which one you get they all have the same problem of room modes that always make the bass lumpy. Way too much one place, way too little the other. Even with EQ, impossible to ever get it right.
Multiple subs however go in multiple locations and produce a lot of different modes in a lot of different locations, the overall effect of which is much smoother, faster, more articulate bass. Its not even close.
So get whichever is cheaper, because then you can afford more, and more is what you want.